cx25840 Conexant Systems, Inc., cx25840 Datasheet - Page 109

• Manufacturer Part Number: cx25840
• Description: Video Decoder And Broadcast Audio
• Manufacturer: Conexant Systems, Inc.

CX25840/1/2/3 Data Sheet
3.20
Table 3-38. AFE Configuration for Sound IF Input
3.20.1
102284B
8/3/05
DROOP_COMP_CH{2:3}
CLAMP_EN_CH{2:3}
VGA_SEL_CH{2:3}
CH_SEL_ADC2
AUD_IN_SEL
Control Bits
Automatic Gain Control
Broadcast Audio Inputs
The CX25840/1/2/3 provides five pins, SIF{1:5}, that can be used to input the sound
IF or AF output of a broadcast tuner. The analog input mux needs to be configured to
route the sound IF signal to the second ADC by setting the appropriate state of the
CH_SEL_ADC2 bit in the Miscellaneous Chip Control Configuration (0x102)
register. If SIF{1:3} inputs are used, analog CH2 must be routed to ADC2. If
SIF{4: 5} are used, analog CH3 must be routed to ADC2.
For the analog channels that are used as SIF inputs, the impedance boosting circuitry
should be disabled through the DROOP_COMP_CH{1:3} bit in the AFE Control 3
(0x106) register. Also the analog clamping circuitry must be disabled by setting the
CLAMP_EN_CH{2:3} bit low in the same register and the AUD_IN_SEL bit set high
in the AFE Control 2 (0x105) register.
In order for the audio decoder to drive the front-end AGC instead of the video decoder
the appropriate VGA gain select bit, VGA_SEL_CH{1:2}, must be set high in the
AFE Control 1 (0x104) register.
bits for sound IF operation.
The analog front-end integrates three differential Variable Gain Amplifiers (VGA):
two for video, and one for audio for scaling the sound IF input. Each of them is
independently controllable. The automatic analog audio gain control function is part
of the demodulator and is used to maximize the SNR performance of the ADC by
optimizing the signal energy to take advantage of the full dynamic range of the ADC.
A running average of the signal level is used to calculate the 6-bit gain setting that is
supplied to the analog front-end VGA. The 2-bit MSBs control the coarse logarithmic
setting, and the 4-bit LSBs control the linear setting.
0 = SIF input on analog CH2
1 = SIF input on analog CH3
Value
0
0
1
1
Conexant
Table 3-38
Miscellaneous Chip Control Configuration: 0x102
summarizes the state of the AFE control
AFE Control 1 and 2: 0x104, 0x105
AFE Control 3: 0x106
AFE Control 3: 0x106
AFE Control 2: 0x105
Register
Detailed Functional Description
3-83